Towards a precise interpretation of the top quark mass parameter in ATLAS Monte Carlo samples

The top quark mass has been in the focus of the theoretical and experimental programmes for many years. Direct measurements profits from kinematic reconstruction of the top decay products after detection, and identity the MC mass with the pole mass which is correct within a precision of ~0.5 GeV. Indirect measurements compare first-principle predictions to unfolded data at parton-level, so a renormalized mass is directly accesible. In this study, we aim to put some light on the interpretation of the MC top mass. The way through MC generators deal with non-perturbative QCD effects may alter the evolution of the MC top mass within the simulation chain, so it gets disconnected from a well-defined top mass from first-principles. Ir order to quantify this, a calibration for the top quark mass parameter in the ATLAS Monte Carlo samples for top quark pair production in $pp$ collisions at $\sqrt{s} =$ 13~\TeV{} is obtained by fitting the particle-level jet mass distribution of large--radius jets containing hadronically-decaying top quark with large transverse momentum, with a calculation at next-to-leading-log precision. The relation between the top mass parameter in the nominal \POWHEG~+~\PYTHIA8 sample and the MSR mass scheme is determined to be: "\Delta^{MSR} = m_{t}^{\text{MC}} - m_{t}^{\text{MSR}} (1~\GeV) = 80^{+350}_{-400}~\MeV" where the uncertainty is dominated by contributions from uncalculated higher orders in the NLL calculation, the fit methodology and underlying event modelling.